Multilayered ceramic capacitors are widely used and their acceptance in a variety of electronic applications and devices is rapidly expanding. Of utmost importance is the desire to increase capability to operate in higher electric fields to enable higher capacitance per unit volume and their expanded use in alternating current (AC) applications.
A disadvantage of multilayer ceramic capacitors is the electrostrictive, or piezoelectric, properties of the ceramic. Ceramics with large electric dipoles such as ferroelectric and anti-ferroelectrics, that are desirable for their high dielectric constants to achieve higher capacitance, have a high degree of electrostriction where these dipoles that can be arranged into domains align with the electric field. At high fields a high degree of alignment occurs through this electrostrictive effect that results in a movement of the MLCC. A high electric field or repetitive application of high fields can result in component failures due to cracking induced by the mechanical stress. This is further compounded in cases where high AC fields are applied that result in a 180° change of direction of dipoles in their respective domains resulting in an oscillatory movement. In severe cases, cracking is observed in the body of the MLCC caused by these stresses, that are also observed in piezoelectric actuators. In some cases the cracks can allow electrical bridging between internal electrodes of opposite polarity which compromises the electrical integrity often leading to failure of the component.
The oscillatory movement of the ceramic can also manifest as microphonic noise wherein the microphonic noise of multiple capacitors may become harmonic and noticeable to users of the electronic equipment. This is particularly undesirable in consumer electronics and especially in electronics intended to have high fidelity such as speakers, headphones and the like.
Due to the problems associated with oscillatory movement in the ceramic the thickness of the active layers is typically high to protect against degradation which limits the capacitance that can be achieved within a given capacitor volume. Therefore, the volumetric efficiency, defined as the capacitance per unit volume, has been limited with MLCC's. This limit is contrary to the well-known and long standing desire to continually miniaturize electronic devices and components therein.
U.S. Publ. Pat. App. No. 2015145343, which is incorporated herein by reference, teaches the use of an open low ceramic density, or sponge layer, as a sacrificial layer between two capacitive stacks to form the body of the capacitor. Though mitigated, oscillation still transmits through the layer of lower ceramic density and, in some cases, may be worse since the stress is transmitted through an area which functions as a pillar between two oscillating planes thereby allowing an oscillation in one plane to impart a point of pressure on the adjacent plane through a strut of the sponge layer. Yet another disadvantage is the manufacturing inconveniences. The stacked layer is formed as a large sheet with a continuous layer of low density ceramic between stacks of capacitive couples. During the manufacturing process the large sheet is diced into small elements. The interface between the capacitive couple and stress layer is prone to degradation due to the necessarily low structural integrity of the low density, or sponge, area. Yet another disadvantage is realized in the ability of processing materials, such as plating material or solder, to infuse into the sponge like stress layer thereby compromising performance of the component.
U.S. Pat. No. 8,576,537, which is incorporated herein by reference, teaches flex mitigation voids extending to the edge of the capacitor wherein any crack propogated by board flexure terminates at the flex void thereby mitigating the crack from propogating into the internal electrodes. The flex propagation voids must be external to the capacitive couple to function and therefore provide no benefit to the mitigation of oscillatory movement in the capacitor between electrodes.
In spite of the efforts of the skilled artisan there is still a need for an MLCC which can withstand higher electric fields and the oscillatory movement of ceramic under high AC voltage without damage to the ceramic or creation of microphonic noise.